The relation between modulation and waveform in communication systems

In recent research efforts related to 5G and beyond radio access technologies, there has been many contributions in the development of new innovative methods and schemes related to the following two research directions at the physical layer level.

• 1) Advanced waveform designs: The purpose here is to create new improved waveforms (other than the conventional OFDM waveform) to enhance the system performance metrics in terms of PAPR, OOBE, ACI, INI, synchronicity, complexity,  spectral efficiency, reliability, and robusetness to ICI, Doppler, and phase noise, etc. Examples of waveforms in this domain include GFDM, FBMC, DFT-s-OFDM, ZT-DFT-s-OFDM, W-OFDM, F-OFDM, edge filtered OFDM, OTFS, OCDM, etc. 

• 2) New modulation techniques: The goal here is to come up with novel modulation schemes (other than the conventional M-ary QAM/PSK modulation schemes) to improve the system performance metrics in terms of spectral efficiency, energy efficiency and reliability.  Examples of techniques in this domain include OFDM-SIM, OFDM-ESIM, OFDM-IM, OFDM-GIM, OFDM-DMIM, OFDM-MMIM, OFDM-SNM, OFDM-SPM, MIMO-SM, MIMO-SSK, etc. 

However, there has been some confusion in the literature in terms of naming, where many people think waveform is modulation as can be seen in [1], which is not very true as they are completely different from each other. To understand the difference and see if there is any relationship between them, we will resort back to the most basic, fundamental definition of each as follows.  

• 1) Modulation means changing the properties of a carrier signal, which can be transmitted and received over a medium, according to the incoming information. For instance, when the used carrier signal in a digital communication system is a sinusoidal function, then its properties including amplitude, phase and frequency can be changed according to the incoming information bits, resulting in what is called ASK, PSK, FSK, respectively. This is well-known in the literature as the most basic form of modulation. As a new front-tier, researchers have recently used the properties of a subblock within an OFDM symbol structure as a new type of carrier at the baseband to send extra information bits.  Basically, the subblock properties in terms of the indices and number of subcarriers within the subblock [2] are used to convey extra information bits. 

• 2) Waveform means the collection of pulses that will be used to carry your modulated data symbols over the channel. These pulses determine all the details related to the time duration, bandwidth, and spectrum shape along with its location in the time-frequency grid to be used during transmission over the channel to protect your data from any kind of interference (ISI, ICI, ACI, INI) and/or other types of distortions (power amplifier distortion and phase noise) while meeting the requirements of the targeted application. For instance, OFDM is a collection of Sinc pulses with certain bandwidth and location in the frequency domain (or summed/multiplexed exponential basis functions with different frequencies), carrying the data modulated symbols. Recently, many different waveforms are proposed such as GFDM, FBMC, DFT-s-OFDM, ZT-DFT-s-OFDM, W-OFDM, F-OFDM, edge filtered OFDM, OTFS, etc. to meet the requirments of future emerging applications in terms of latency, interference, synchronicity and immunity to impairments. 

From the above discussion and explanation, the differences between waveform and modulation concepts are clearly stated and concluded. This description is expected to help researchers choose appropriate terms and names to the proposed schemes, methods, techniques, etc. according to their exact functionality to avoid any confusion in the basic understanding of new concepts.

[1] Y. Cai, Z. Qin, F. Cui, G. Y. Li and J. A. McCann, "Modulation and Multiple Access for 5G Networks," in IEEE Communications Surveys & Tutorials, vol. 20, no. 1, pp. 629-646, Firstquarter 2018.

[2] A. M. Jaradat, J. M. Hamamreh and H. Arslan, "Modulation Options for OFDM-Based Waveforms: Classification, Comparison, and Future Directions," in IEEE Access, vol. 7, pp. 17263-17278, 2019.

[3] J. M. Hamamreh and H. Arslan, "Time-frequency characteristics and PAPR reduction of OTDM waveform for 5G and beyond," 2017 10th International Conference on Electrical and Electronics Engineering (ELECO), Bursa, 2017, pp. 681-685.

Author contact: jehad.hamamreh@gmail.com